Einstein's 'impossible hope' comes true as scientists weigh star with gravity

Updated 2017-06-08 09:28:57 Xinhua

A little more than 100 years after Albert Einstein developed his theory of general relativity, astronomers have successfully used its laws to determine the mass of a distant star, a feat the iconic scientist had thought "impossible."

In a study published Wednesday by the U.S. journal Science, researchers scanning the skies with the Hubble Space Telescope revealed how the gravity of a white dwarf, the burned-out remnant of a normal star, warps space and bends the light of a distant star behind it, allowing them to measure a star's mass with gravity for the first time.

"Einstein would be proud," Terry Oswalt of Embry-Riddle Aeronautical University, wrote in a related perspective piece in Science. "One of his key predictions has passed a very rigorous observational test."

One of the key predictions of general relativity set forth by Einstein in 1915 was that whenever light from a distant star passes by a closer object, gravity acts as a kind of magnifying lens, brightening and bending the distant starlight.

When a star in the foreground passes exactly between us and a background star, Einstein predicted, such a phenomenon, called gravitational microlensing, results in a perfectly circular ring of light -- a so-called "Einstein ring."

The first evidence of the bending of light came in the form of an eclipse in 1919, providing one of the first convincing proofs of Einstein's general theory of relativity.

But Einstein also predicted that if the two stars are just out of alignment, it would cause the background star to appear off-center in a way that could be used to directly determine the mass of the foreground star.

Yet, in a 1936 article in Science, he added that because stars are so far apart "there is no hope of observing this phenomenon directly."

In the current study, an international research team directed by Kailash Sahu of the Space Telescope Science Institute in Baltimore took advantage of the superior angular resolution of the Hubble Space Telescope and proactively searched more than 5,000 stars for such an asymmetric alignment.

They realized that the white dwarf Stein 2051 B was set to be in such a position in March 2014 and then directed the Hubble Space Telescope to observe the phenomenon, measuring tiny shifts in the apparent position of a background star behind it.

Based on the data, the researchers estimated the white dwarf star's mass to be roughly 68 percent of that of our sun.

"The research by Sahu and colleagues provides a new tool for determining the masses of objects we can't easily measure by other means," Oswalt said.

For the average star-gazer, Oswalt added, the findings are meaningful because "at least 97 percent of all the stars that have ever formed in the Galaxy, including the Sun, will become or already are white dwarfs -- they tell us about our future, as well as our history."

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